The proper functioning of a joint, such as the knee, hip, shoulder, ankle or elbow can be impeded by a variety of factors, including, disease, such as osteoarthritis, mechanical injury, bone deformation and a variety of other factors. Arthroplasty, or the surgical restoration of a joint, j is a known procedure that is often used to relieve pain and improve joint function by replacing the diseased or damaged articulating surfaces of a joint with prosthetic components. Achieving joint balance is a primary goal for arthroplasty surgeons. A balanced joint is a joint that has the proper articulation in all orientations of the joint. The patient may be most comfortable when the artificial joint replicates the kinematics of the original, natural joint.
One of the most common arthroplasty procedures is knee replacement surgery. Some common forms of knee replacement surgery include total knee replacement (“TKR”) surgery; partial knee replacement surgery, which is also known as unicompartmental arthroplasty (“UKA”); and revision knee surgery; each of which is briefly described below.
Generally, in a TKR, the femur's lateral and medial condyles, or the articulating surfaces at the femur's distal end, are removed and replaced with a femoral prosthetic component. Additionally, in a TKR, the tibial plateau at the tibia's proximal end is also removed and replaced with a tibial prosthetic component.
In contrast, during a UKA, the knee is generally divided into three compartments—namely a medial compartment that is located at the inside of the knee, a lateral compartment that is located at the outside part of the knee, and a patellofemoral compartment that is located between the kneecap and the femur. In a UKA where the damage is confined primarily to one compartment (namely the medial or lateral compartment), the articulating surfaces from that particular compartment of the femur and/or tibia are usually removed and replaced with prosthetic components.
With respect to revision knee surgery, such surgeries generally involve removing one or more prosthetic components that were previously placed within the knee (“primary components”) but have become worn, did not fit properly, or have otherwise prevented the knee from functioning properly. The primary components are then typically replaced with one or more replacement components (“revision prosthetic components”).
Joint replacement surgery involves exposing the joint, preparing the surfaces, correcting any misalignment, and creating the appropriate tension on the constraining ligaments to allow the components to move through a smooth balanced arch of motion. If there is lack of tension holding one side of the joint together compared to the amount of tension on the other side, this creates an imbalance and an opening of the loose side of the joint when moves through its range of motion. A patient can often feel this extra motion, laxity, and/or imbalance as a sense of instability in the joint and it can contribute to patient dissatisfaction as well as a joint that wears out faster as it is not restrained to the smooth motion it was engineered to follow.
A surgeon needs to balance the space into which he is placing a component so that the medial space will match the lateral space, in both the flexed and extended positions of the joint. This will result in equal contact pressures throughout the joint as it is moved back and forth through flexion and extension. The surgeon can alter the shape of the exposed bone surface, tighten or loosen the ligamentous restraint on each side of the joint, or alter the size of the components he is using to replace the joint.
Techniques currently used to balance the joint have significant drawbacks. For example, in some circumstances, surgeons will cut away or remove more bone from the tight side of the joint to open this space to match the space on other side. Another technique is to release or stretch the tensioning ligament on the tight side of the joint to allow it to open up and match the looser side. With these cut or release techniques there is always the risk of removing too much bone or over releasing the ligament to the point of failure. These problems can be difficult to correct, and can result in increased surgical and recovery time. Furthermore, a wider range of joint components may be needed to accommodate these iatrogenic conditions.